For many years, the pharmaceutical industry has been developing and discovering suspending agents useful in the preparation of pharmaceutical suspensions. Such suspensions are efficacious for the delivery of therapeutic agents and other uses. These suspensions can be used in a wide variety of applications such as parentral, topical, oral, rectal or the like and, of particular importance to the present invention, ophthalmic, otic and nasal. Examples of such suspensions are described in U.S. Pat. Nos. 7,001,615; 6,359,016; 6,284,804; 6,139,794; 5,932,572; 5,461,081 and US Patent Publication Nos. 20060257487; 20060257486; 20060122277; 20030139382; 20020037877; all of which are incorporated herein by reference for all purposes.
Generally speaking, it is desirable for suspending agent to assist in maintaining a therapeutic agent suspended within a suspension (e.g., an aqueous suspension) for a relatively large amount of time without allowing the therapeutic agent to settle out of the suspension. However, many popular conventional suspending agents allow therapeutic agent to settle out of suspension rather quickly. Moreover, many popular suspending agents also allow the therapeutic agent to become relatively tightly packed within the suspension and may not allow the therapeutic agent to be easily re-suspended. As examples, non-ionic polymers such as hydroxypropyl cellulose and hydroxyethyl cellulose often allow the therapeutic agent to settle out of solution at undesirably high rates and allow the therapeutic agent to become tightly packed once settled.
In addition to the above, many conventionally used suspending agents have been found to be incompatible with ingredients that have recently become desirable within pharmaceutical compositions. As one example, in the ophthalmic industry, there has been a move toward antimicrobial agents such as polymeric quaternary ammonium compounds that exhibit relatively low toxicity, however, certain anionic suspending agents such as carbopol, xanthan gum and carboxymethyl cellulose can be incompatible with such antimicrobial agents under certain circumstances.
In view of the above, there is a need for a suspension and suspending agent that assist the therapeutic agent in remaining suspended in an aqueous or other environment and/or assist the therapeutic agent in resisting tight packing upon settling out of the suspension. Additionally or alternatively, there is a need for suspending agent that exhibits a high degree of compatibility with highly desirable low toxicity ingredients of the suspensions.